Preemptive use of GCV remains the method of choice in the prevention of CMV disease following allogeneic BMT. In a recently completed Phase I/II study (Boeckh et al.) oral GCV was used for CMV prophylaxis starting at day 35 through day 100 post-transplant. The bioavailability was comparable to other patient populations even in the setting of GI acute GVHD (7.2% vs 6.9%). However, GI intolerance was seen in six of 21 patients (29%) when GCV was started at day 35. The authors suggested starting oral GCV during the mainenance treatment after an initial course of IV GCV. Because the onset of CMV reactivation is usually between the 5th to 9th week post BMT (days 42 to 63) we want to start those patients with CMV reactivation on a course of therapy. This will include 1 week of intravenous GCV at 5mg/kg/bid, which is our standard induction regimen, followed by a 5-week course of oral GCV at doses previously determined in the Phase I/II study conducted at our center of 1000mg of GCV three times a day. Steady state pharmacokinetic profiles will be determined as well.